Production of oil shale by in situ pyrolysis



y 4, 1968 R. B. NEEDHAM 3,382,922

PRODUCTION OF OIL SHALE BY IN SITU PYROLYSIS Filed Aug. 31, 1966 N Qk 2:0: 62.55 mmouwm 609 b m2; ow om ow om om o. o

19d HIVHS HO :!O HLONBELLS BAISSBHdWOD O00 O00 00h INVENTOR R. B. NEEDHAM l 4 19d HlDNI-IELLS HAISSEHdWOD 2 f A 7' TORNE VS United States Patent 3,382,922 PRODUCTION OF OIL SHALE BY IN SlTU PYROLYSIS Riley B. Needham, Bartlesville, 0kla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Aug. 31, 1966, Ser. No. 576,271 4 Claims. (Cl. 166-11) This invention relates to a process for the in situ production of oil from oil shale by pyrolysis with hot gases.

Tremendous deposits of oil shales occur in Colorado, Utah, and Wyoming, and various petroleum companies and the Federal Government are doing research on methods of producing oil from these deposits. Numerous proposals have been made, including mining the shale and retorting the mined shale above ground and applying heat to the shale in situ with hot gases including oxygen and excluding oxygen. Steam, hot combustion gas, hot air, etc, have been proposed as heating media for the pyrolysis operation. One of the problems encountered in the in situ production of oil shales with hot gases lies in the in permeability of the shale, which drastically limits the contact area between the hot gas and the shale to the wall of the well or mine shaft thru which the gas is injected. To overcome this problem, conventional hydraulic fracturing has been resorted to to open up fracture surfaces between wells thru which hot gases can be passed. After fracturing the shale between wells, a propping agent has been introduced to the fractures to hold the fractures open for the pyrolysis procedure. While the shale is at normal temperatures, the propping agent is effective in holding the fractures open but, as the shale is raised in temperature by the injected hot gas, it becomes plastic or relatively soft compared with its cool condition so that the weight of the overburden on the propping agent causes the propping agent to become embedded in the fracture surfaces, thereby allowing the fractures to close and shut off the flow of gas.

U.S. Patent 2,969,226 discloses a method of overcoming the foregoing problem which comprises maintaining the pressure of the injected pyrolysis gas equal to or slightly greater than the rock pressure (overburden pressure), which has the effect of holding the fractures open during the pyrolysis operation. This procedure requires operating at continuously high pressure, the pressure depending upon the depth of the shale formation, during the life of the production operation.

This invention is concerned with a process for pyrolyzing oil shale to produce oil therefrom by contacting the shale adjacent fractures therein with hot gases, which permits operation during the major portion of the life of the process at lower pressures than conventional.

Accordingly, it is an object of the invention to provide an improved process for producing shale oil from an oil shale by in situ pyrolysis with hot gases. Another object is to provide an oil shale pyrolysis process which is operable at more economical, lower pressures than are utilized in conventional processes of this nature. A further object is to provide a process for the pyrolysis of oil shale which is adaptable to various types of shales. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises contacting the fracture surfaces of oil shale with hot pyrolyzing gases injected thru an input well and produced thru an output well, utilizing a propping agent and only sufficient pressure to prevent embedment of the propping agent in the fracture surfaces during the initial phase of the operation while pyrolyzing and hardening the fracture surfaces and adjacent shale and thereafter continuing the pyrolysis at substantially less pressure than propping pressure. Oil shales have different oil contents ranging from S or 10 gallons per ton to more than gallons per ton. The plasticity of the shale when heated to pyrolysis temperatures is proportional to the oil content of the shale. When an oil shale has an oil content of less than 20 gallons per ton of shale, it is produced with a hot gas pressure only sufiicient to provide adequate flow rates of gas thru the fracture without maintaining substantial backpressure on the output or production well during both phases of the process. When the shale has an oil concentration in the range of about 20 to 35 gallons per ton of shale, an elevated pressure, less than propping pressure but sufiicient to prevent closing of the fracture by embedding of the propping agent, for example, a pressure in the range of A to of propping pressure, is utilized during the initial phase of the operation. With oil shales having an oil concentration greater. than about 35 gallons per ton of shale, a pressure at least as great as propping pressure is utilized during the initial phase of the operation with much lower pressures being utilized after the shale is hardened along the fracture surfaces.

The invention is based upon the fact that oil shales can be hardened during pyrolysis so that they will resist ernbedding of the propping agent in the fracture surfaces even at high pyrolysis temperatures of 1000 F. and higher. Thus, the process is operated in two distinct phases, the first effecting hardening of the shale fracture surfaces during pyrolysis and the second effecting pyrolysis at a higher and more efficient temperature at lower, more economical pressures.

Oil shales can be pyrolyzed and hardened at temperatures of 500 F. and up to 650 F. while maintaining the strength of the shale, particularly with shales containing not more than 35 gallons of oil per ton of shale. After the initial pyrolysis phase at relatively low temperature which effects hardening of the shale, the pyrolysis gas is raised substantially in temperature to above 800 F. and, preferably, above 900 F. up to 1000 F. or higher. The gas used in the latter step can contain a substantial but minor concentration of free oxygen to effect combustion and further heating of the shale.

In order to provide a better understanding of the invention, reference is made to the accompanying drawing of which FIGURE 1 shows compressive shale strength of two shales under different pyrolysis temperatures and FIGURE 2 presents a curve showing the compression strength of an oil shale of specific oil content when heated to 700 F. for increasing periods of time.

Referring to FIGURE 1, curve A represents the com pression strength at different test temperatures of a Green River (Colorado) shale which assays 20 gallons of oil per ton of shale by Fischer assay. Curve B shows the strength at different temperatures of oil shale assaying 35 gallons of oil per ton of shale. In the tests for both FIGURES 1 and 2, an oil shale core 1 inch in diameter and 1 /2 inches in length was loosely enclosed within an upright stainless steel tube between pressure blocks across the tube providing an annulus around the core for circulation of hot gases. Hot pyrolyzing gas was injected near the bottom of the annulus surrounding the core and vented opposite the injection point and near the top of the an nulus to provide good circulation of the pyrolyzing gas around the core. The heating rate was controlled at F. per hour and a core was held at test temperature for 2-3 hours before testing for compression strength.

Referring to curve A, it can be seen that, as the temperaiure rose to 600 F., the strength of the shale was about 8800 psi. and was rapidly decreasing until it reached a minimum just above 4000 p.s.i. at about 670 F. so that, with continued temperature rise, the shale fairly rapidly regained strength up to the maximum tested. It is clearly demonstrated by curve A that an oil shale with an oil content of gallons per ton can be raised in temperature to eflicient pyrolyzing gas temperatures upwards of 800 F. without decreasing the compression strength to the extent that the propping material will become embedded in the shale at depths up to about 4000 feet. It is well known that the pressure at any given depth is about 1 p.s.i. for every foot of depth. Oil shales occur at relatively shallow depths and, therefore, any known shale could be pyrolyzed at relatively low gas pressure at increasing temperatures without closing the fracture thru which the gas is passed.

Referring to curve B, it can be seen that the compressive strength of a shale which assays gallons per ton rather sharply decreases as it is heated, particularly in the range of 600-700 F., to a minimum of about 250 psi. before increasing to a strength of about 2800 psi. at 1000 F. Thus, curve B clearly demonstrates that substantial pyrolyzing gas pressure must be used between about 650 and 900 P. if the temperature of the gas is raised at the rate of 150 F. per hour with holding for 20 hours at each hundred-degree temperature level. Hence, with this type of shale, which is at the maximum of the 20-35 gallons per ton range, an elevated pressure less than propping pressure but sufficient to prevent closing of the fracture by embedding of the propping agent in the shale is effective when pyrolyzing shales in this category. The gas pressure utilized increases as the oil concentration in the shale increases so that a considerably lower pressure is effective with an oil shale which assays 20* gallons per ton as compared to the oil shale of curve B assaying 35 gallons per ton. The latter requires gas pressure approaching but still below propping pressure to maintain the fracture open and overcome embedding. It is also apparent from a consideration of curve B that shales which assay over 35 gallons per ton require propping pressure during the hardening phase of the operation.

Referring to FIGURE 2, which represents data applying to an oil shale assaying 27 gallons of oil per ton of shale, it can be seen that the compressive strength of the shale when brought to 700 F. at the rate of 150 F. per hour decreases fairly rapidly with time of heating at 700 F. to about 30 hours, at which time the strength of the shale is rising so that, after 65 hours at 700 F., the strength of the shale is about 2400 psi.

Shale tested to provide the curve of FIGURE 2 can be treated at a temperature in the range of 500-600 F., or even to 650 F., to produce carbonization and hardening which reaches a minimum softening point, as measured by compression strength, substantially higher than the 500 p.s.i. of FIGURE 2. To illustrate, at a temperature of 550 P. less oil is produced from the shale and more oil is carbonized to provide higher strength but the carbonization process which hardens the shale takes a considcrably longer period of treatment, the length of treatment increasing with decreasing temperatures. Hence, it is more efiicient to utilize as high a carbonizing or pyrolyzing temperature as consistent with maintaining the minimum compressive strength at the desired level. A compromise is made among minimum compressive strength, time of treatment, and the pressure of the pyrolyzing gas. In applications where it is desirable to pyrolyze at higher than minimum temperatures to hasten the hardening process, pyrolyzing gas pressure is elevated to compensate for the relatively lower minimum compression strength resulting from higher temperature treatment.

The lower pressures utilized or practiced in the second phase of the pyrolysis operation permitted after treatment in the first phase, in which the fracture surfaces are hardened, contributes materially in an economical way to the efliciency of the process.

Compressive strengths of Green River oil shales with Fischer assays from 10 to 65 gallons per ton (g.p.t.) were measured at temperatures from 75 F. to 1200 F. Representative results obtained up to 500 F. are tabulated below.

Temper- Shale Compressive Strength (p.s.i.)

ature 10 g.p.t. 20 g.p.t. a0 g.p.t. 40 g.p.t. g.p.t. e5 g.p.t.

23, 900 19, 400 17, 300 16, 000 14, 800 13, 200 6, 500 9, 700 7, 800 7, 000 Plastic Plastic 28, 000 8,000 5, 200 4, 000 3, 400 2, 800

At 700 F. and above, compressive strengths became dependent upon time at a given temperature, first decreasing and then increasing. For example, at 700 F., the compressive strength of a 27 g.p.t. shale dropped from 3,100 psi. at the end of one hour thru a minimum of 500 psi. at 27 hours, then increased to 2,400 psi. at hoursthe end of the test period.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for producing oil from subterranean oil shale in situ comprising the steps of (a) establishing gas flow thru at least one fracture at an intermediate level in said shale between at least a pair of Wells therein;

(b) providing a sufiicient amount of particulate propping agent in said fracture to prop same open;

(c) passing hot gas substantially free of 0 at a temperature of at least 500 F. thru said fracture from one of said wells to another as a production well for a sufficient time to pyrolyze and harden the pyrolyzed area adjacent each fracture surface and produce oil therefrom, utilizing a selected gas pressure,

(1) when said shale has an oil content of less than 20 gallons per ton of shale, only sufficient to provide adequate flow thrus the fracture without substantial backpressure on said production well;

(2) when said shale has an oil con-centration in the range of about 20 to 35 gallons per ton of shale, an elevated pressure less than propping pressure but sufficient to prevent closing of said fracture by embedding of said propping agent; and

(3) when said shale has an oil concentration greater than about 35 gallons per ton of shale, at least propping pressure to hold said fracture open until hardening is completed;

(d) following step (c), passing hot gas thru said fracture at pressures substantially lower than propping pressure and at a higher temperature than that of step (c) to extend the pyrolysis zone and produce additional oil; and

(e) recovering the produced oil from said production well.

2. The process of claim 1 wherein the temperature in step (c) is in the range of 500 to 900 F. and the temperature in step (d) is at least 1000 F.

3. The process of claim 1 wherein said shale has an 65 oil concentration in the range of about 20 to 35 gallons per ton, the gas in step (c) is at a temperature in the range of about 500 to 700 F. and a pressure in the range of A to of propping pressure, and the temperature in step (d) is above 900 F.

4. The process of claim 3 wherein the heating gas in step (d) contains a substantial but minor concentration of free 0 to effect combustion and further heating of the shale.

(References on following page) Marx et a1. 16611 Huntington 16611 Closrnann et a1. 16611 Gilchrist 16611 Nichols 16611 Gilchrist 16611 Huntington 16611 X Thomas 16611 X Reistle 16640 X Prats 16611 Slusser 166-11 X STEPHEN J. NOVOSAD Primary Examiner. 

1. A PROCESS FOR PRODUCING OIL FROM SUBTERRANEAN OIL SHALE IN SITU COMPRISING THE STEPS OF (A) ESTABLISHING GAS FLOW THRU AT LEAST ONE FRACTURE AT AN INTERMEDIATE LEVEL IN SAID SHALE BETWEEN AT LEAST A PAIR OF WELLS THEREIN; (B) PROVIDING A SUFFICIENT AMOUNT OF PARTICULATE PROPPING AGENT IN SAID FRACTURE TO PROP SAME OPEN; (C) PASSING HOT GAS SUBSTANTIALLY FREE OF O2 AT A TEMPERATURE OF AT LEAST 500* F. THRU SAID FRACTURE FROM ONE OF SAID WELLS TO ANOTHER AS A PRODUCTION WELL FOR A SUFFICIENT TIME TO PYROLYZE AND HARDEN THE PYROLYZED AREA ADJACENT EACH FRACTURE SURFACE AND PRODUCE OIL THEREFROM, UTILIZING A SELECTED GAS PRESSURE, (1) WHEN SAID SHALE HAS AN OIL CONTENT OF LESS THAN 20 GALLONS PER TON OF SHALE, ONLY SUFFICIENT TO PROVIDE ADEQUATE FLOW THRUS THE FRACTURE WITHOUT SUBSTANTIAL BACKPRESSURE ON SAID PRODUCTION WELL; (2) WHEN SAID SHALE HAS AN OIL CONCENTRATION IN THE RANGE OF ABOUT 20 TO 35 GALLONS PER TON OF SHALE, AN ELEVATED PRESSURE LESS THAN PROPPING PRESSURE BUT SUFFICIENT TO PREVENT CLOSING OF SAID FRACTURE BY EMBEDDING OF SAID PROPPING AGENT; AND (3) WHEN SAID SHALE HAS AN OIL CONCENTRATION GREATER THAN ABOUT 35 GALLONS PER TON OF SHALE, AT LEAST PROPPING PRESSURE TO HOLD SAID FRACTURE OPEN UNTIL HARDENING IS COMPLETED; (D) FOLLOWING STEP (C), PASSING HOT GAS THRU SAID FRACTURE AT PRESSURES SUBSTANTIALLY LOWER THAN PROPPING PRESSURE AND AT A HIGHER TEMPERATURE THAN THAT OF STEP (C) TO EXTEND THE PYROLYSIS ZONE AND PRODUCE ADDITIONAL OIL; AND (E) RECOVERING THE PRODUCED OIL FROM SAID PRODUCTION WELL. 